\documentclass{standalone}
\usepackage{tikz}
\usepackage{ctex,siunitx,bm}
\setCJKmainfont{Noto Serif CJK SC}
\usepackage{tkz-euclide,ninecolors}
\usepackage{amsmath}
\usetikzlibrary{patterns, calc}
\usetikzlibrary {decorations.pathmorphing, decorations.pathreplacing, decorations.shapes,}
\begin{document}
\small
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  \fill[azure7](0,2.0)--++(150:0.1)--(-1.5,1.9)node[text=black,below left=1mm]{$S$};
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  \draw[->](-0.5,2.7)--(0.5,2.7)node[midway,above]{$I$};
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\end{tikzpicture}
\end{document}